The word for today is…
touchstone (noun) –
1 : a fundamental or quintessential part or feature
2 : a test or criterion for determining the quality or genuineness of a thing
3 : a black siliceous stone related to flint and formerly used to test the purity of gold and silver by the streak left on the stone when rubbed by the meta
Source : Merriam -Webster
Etymology : Since the early 16th century, touchstone has referred to a particular kind of siliceous stone (that is, stone containing silica) used to do a particular job: determine the purity of precious metals. The process involves comparing marks made by rubbing a sample of a metal of known purity to marks made by a metal of unknown purity. The method is accurate enough in the case of determining the purity of gold that it is still in use today. Figurative use extended from this literal use, with touchstone functioning as a word for a test or criterion to determine the quality of a thing, and later to refer to a fundamental or quintessential part or feature of something. The more general meaning of quality (e.g. Good service is one touchstone of a first-class restaurant.) is a much later use of the word.
If you enjoyed this BFD word of the day please consider sharing it with your friends and, especially, your children.